SCCP (Signalling Connection Control Part)

Questionnaire on SCCP:

1.) What are the benefit of SCCP over the MTP, which protocols uses the SCCP.
2.) Define the SCCP architecture and functional Area.
3.) Why we use XUDTS/UDTS messages.
4.) Define the connection oriented Message.
5.) Define the Connection Less messages.
6.) define SCCP Message structure.
7.) structure of CR,CC, CREF, UDT,UDTS,DT ?
8.) define GT and GTT?
9.) What is AI?
10.) Explain the User Data and Segmentation.
11.) What is TI value and TI Flag.

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Answers:

1.) Benifits of SCCP over the MTP:

- Provide the Segmentation and reassemble when message is large for 1 MSU.
- Provide more flexible routing using the Global Tittle (GT)
- Used for both Connection-less and connection-oriented data transfer.
- Provide management and addressing of Subsystem.

Uses:

- SCCP is used extensively in cellular networks. BSSMAP and DTAP use it to transfer the radio related messages in GSM.
- In conjection with the TCAP, SCCP is also used throughout the GSM NSS to transport MAP signaling between the core GSN components.

2.) SCCP functional areas:
- SCCP Connection-oriented Control (SCOC) - responsible for setting up and releasing the virtual connection between two SCCP users
- SCCP Connection-less control (SCLC) - responsible for transferring the data b/w the users without using the virtual connection. Primarly used by TCAP.
- SCCP Routing Control (SCRC) - provide the routing beyond the MTP, through the use of the global title.
- SCCP Management (SCMG) responsible for tracking the application status and informing the SMG at other SCCP node.

Application that uses the service of SCCP are called Subsystems.

Classes of service:

• Class 0—Basic connectionless class - it has no sequencing control. i does not impose any condition on SLS, therefore SCCP message can be delivered in out-of-sequece.
• Class 1—In-sequence delivery connectionless class - it adds the sequence control to class 0 service by requiring to insert the same SLS to all NSDU.

• Class 2—Basic connection-oriented class - Assign the local reference numbers (SLR,DLR) to create logical connection. it does not provide the flow control, loss, and mis-sequence detection.
• Class 3—Flow control connection-oriented class - Class 3 is an enhanced connection-oriented service that offers detection of both message loss and mis-sequencing

3. ) UDTS and XUDTS used for the Negative Ack for the UDT and XDT respectivily. It indicates to the originating SCCP that a UDT message that is sent cannot be delivered to its destination.

4.) Connection Oriented Messages: CR, CC, CREF, DT1, DT2, RLSD, RLC, AK ( Data Ack), ED (expedited Data), EA (expedited Data Ack), RSR (Reset Request), RSC (Reset Confirm), IT (Inactivity Test).

5.) Connection Less Messages: UDT, UDTS, XUDT, XUDTS, LUDT, LUDTS.

6.) Apart from CIC, SCCP message structure is same is ISUP messages. Refer the http://telecomprotocols.blogspot.com/2012/09/sccp-messages.html for the complete coverage of sccp message.

- mandatory Fixed Part: because it mendory and are of fixed length so length required. type and order is known so no parameter name required (only value is required). Usally Message Type, SLR, and Protocol Class are in MF Part.

  - Mandatory variable Part: only Length and value are required. No Tag required.   Usally Called Part Address is in MV Part.

 - Optional Part: Tag, Length and Value are required. One Octet END of Optional Parameter field is placed at end of last optional parameter. it is simply coded as all zeros. Usally Calling Party Address, etc in Optional Part.

7.)  Connection Request:
- Message Type
- SLR
- Protocol Class
- Called Party Address
-  Calling Party Address
- MCC/MNC/LAC/Cell ID/IMSI

Connection Confirm:
- Message Type
- SLR
- DLR
- Protocol Class
- Called Party Address

Connection Refused (CREF):
- Message Type
- DLR
- Refusal Cause
- Called Party Address
- Data

Released (RLSD):
 - Message Type
- DLR
- SLR
- Refusal Cause
- Called Party Address
- Data

Release Complete (RLC):
- Message Type
- SLR
- DLR

Data Form1 (DT1)
- Message Type
- DLR
- Segmentation/Reassembling
- Data

Unit Data ( UDT):
- Message Type
- Protocol Class
- Called Party Number
- Calling Party Number
- Data

Unit Data Service ( UDTS ):
- Message Type Code
- Return Cause
- Called Party Address
- Calling Party Address
- Data

8.) A global title is an address, such as dialed-digits, which does not explicitly contain information that would allow routing in the SS7 network. A GT is a telephony address. As such, the GT address must be translated into an SS7 network address (DPC+SSN) before it can be finally delivered.

GTT is an incremental indirect routing method that is used to free originating signaling points from the burden of having to know every potential application destination (that is, PC+SSN).

If the GTT function results in a "routing indicator" equal to "Route on GT", then the GTT function must provide a global title and the DPC of the SCCP node where that global title will be translated. This process shall be repeated until the GTT function results in a "routing indicator" equal to "Route on SSN", which means that the final destination has been determined.

9.) Address Indicator (AI): AI is the first field within CgPA/CdPA and is one octet in length. Its function is to indicate which information elements are present so that the address can be interpreted.

| Network Indicator (1 bit) | Routing Indicator (1bit) | GT Indicator (4bit) | SSN Indicator (1bit) | PC Indicator (1bit)|
GT Indicator:
(0000 = GT Not Included,
0001 = GT Includes Nature of Address,
0010 = GT Includes Translation Type,
0011 = GT Includes Translation Type, NP, and Encoding Scheme),
0100 = GT Includes TT, NP, ES, and NOA Indicator)
Routing Indicator:
- Route on GT
-Route on PC +SSN

10.) The data (from subsystems) is sent in information elements called Network Service Data Units (NSDUs). SCCP provides the capability to segment or reassemble an NSDU that is too large to fit in a single MTP message (MSU) so that it can be transmitted over a number of MSUs (16 maximum).
When using the connectionless classes, if an NSDU is greater than 255 octets when using a UDT message or 254 when using a XUDT message, the originating node splits the NSDU into a number of XUDT messages.

If an NSDU is greater than 255 octets when using the connection-oriented classes, the originating node splits the NSDU into several DT messages.

11.) The combination of TI value and TI flag is used to uniquely identify the separate transaction. It is widely important in Multiparty call, call waiting, call hold where more than one call established at a time.

TI (Transaction Identifier) Flag: This is flag to identifies the originator of the transaction.
For example flag 0 is used for the message is sent from the site that originates the TI.
while flag 1 used for the message is sent to the site that originates the TI.

TI (Transaction Identifier)Value: This is value that has been assigned by the originator of transaction at the start of transaction to uniquely identify the separate transactions. A available TI value is assigned to this transaction and it remains fixed to this transaction for its lifetime and made free while the transaction ends.

For Example for the scenario: Mo-A calls Mo-B. Mo-B is having call hold and call waiting. Mo-C calls to Mo-B. Mo-B holds the Mo-A and retrieve the Mo-C. Mo-B disconnects Mo-C and retrieve Mo-A. And Mo-B Disconnect Mo-A.

The TI value and Flag should be as follows for the above scenario:

Mo-A  {0,0 (TI flag,value)}              Calls             Mo-B {1,0 ( TI flag,value)}

Mo-C  {0,0 ( TI flag,value)}              Calls            Mo-B {1,1 ( TI flag,value)}


Mo-B {1,0 ( TI flag,value)}              Holds           Mo-A {0,0 (TI flag,value)}

Mo-B  {1,1 ( TI flag,value)}            Connects      Mo-C {0,0 (TI flag,value)}

Mo-B  {1,1 ( TI flag,value)}            Disconnect     Mo-C {0,0 (TI flag,value)}

Mo-B {1,0 ( TI flag,value)}              Retrieve        Mo-A {0,0 (TI flag,value)}

Mo-B {1,0 ( TI flag,value)}              Disconnect     Mo-A {0,0 (TI flag,value)}